1. Field of the Invention
The present invention generally relates to a packet communication network composed of transmission lines for connecting a plurality of terminals accomodated in a packet -switching node and transmission lines for connecting a plurality of packet-switching nodes and, more particularly, to a packet transfer regulating apparatus for alleviating congested states of packet-switching nodes and transmission lines.
2. Description of the Related Art
A packet communication network is comprised of transmission lines for connecting terminals accomodated in a packet -switching node and transmission lines for connecting packet-switching nodes with each other. The terminal is a source for generating a wide variety of information such as data, image data, audio data and so on. The packet-switching node is adapted to extract priority information contained in a packet from a terminal and to add a new transfer destination address to the header of the priority information to form a packet format or to judge a route to which data is transferred.
In a packet communication network, the packet-switching node extracts from a probability standpoint communication information generated by the terminal and converts it to packet data to effect the communication. Thus, highly efficient resource administration is realized on the basis of large grouping effect within the network. Respective packets are temporarily stored in a transfer queuing buffer provided within the node, thereby keeping constant the band width of the transmission line.
However, when a synchronization communication or the like whose real time property is required is performed by using packet data, information generated from the terminals is irregular. Thus, if respective data are multiplexed on the line, there is a low probability that packet data will be concentrated on the packet-switching node. As a consequence, packets cannot be transferred within the node, and delay in transmission of a communication packet within the node is increased. Then, if the number of packets (stored packets) accumulated in the transfer queuing buffer exceeds an allowable limit, the packet-switching node or the transmission line is placed in a congested state, and packets are discarded in the packet-switching node or in the transmission line. Thus, there arises the problem that packet communication quality is deteriorated.
Such a congested state may occur in the transmission line or in the packet-switching node. When a congested state occurs, a control operation is performed so that the flow of packets to the congestion occurrence point is regulated (stopped) and the flow of packets is regulated so that it becomes less than the flow of packets out of the congestion occurrence point. Thus, excess packets are directed to another switching-node. This control operation is what might be called a transfer regulation. The following two points are considered as points at which the transfer regulation is executed:
(1) An output point of a packet-switching node in which traffic can be flowed to the congestion occurrence point through the transmission line; and
(2) A traffic input point in which traffic is flowed to a network (packet communication network).
Ideally, the input point at which excess traffic, which causes congestion, is input to the network is detected at the congestion occurrence point. The input is then regulated at that input point without delay (zero time). However, a regulation instruction is transmitted to the input point with a constant delay time. Thus, it is impossible to regulate traffic flowing during that period of time. This means that the system for directly regulating the input point cannot be realized in a large-scaled network.
Concentration of traffic is not limited to the excess input of one input (traffic), but may occur when several inputs are accidentally overlapped. When this occurs, the overlapped state may cease to exist automatically after a certain excessive state. Accordingly, the system for regulating traffic flowing to the input point of the network is not always optimum.
When the concentration of traffic occurs in a wave-like fashion, the following transfer regulating system is proposed. Regulation at the point indicated by (2) above is mainly performed and, as indicated by (1) above, the output point of the packet-switching node immediately before the congestion occurrence point is regulated and such regulation is sequentially extended to nodes of preceding stages.
The following three conditions are indispensable to such a transfer regulation system:
(a) [short regulation achieving time]
With ideal transfer control, regulation reaches the input point with a physical transmission time ranging from the congestion occurrence point to the input point in which the traffic is input to the network. In practice, however, the regulation achieving period is affected by the amount of excess traffic flowing into the network. This increase brings about an increase in the delay time of other communication and an increased buffer amount at the congestion occurrence point. Therefore, the regulation achieving time must be minimized.
(b) [minimum regulation extending or covering range]
A system for regulating only a communication (traffic) which causes congestion is ideal because invalid and excessive regulation occurs when it is extended to communications other than that which passes through the congested transmission line.
(c) [appropriate hardware amount]
The amount of hardware must be kept within practical limits.
There are two systems based on conventional techniques.
The first is a direct regulation system. In this system, an input source in which an excessive flow occurs at a congestion occurrence point is specified and a regulation instruction is issued directly to the input source, and in this system, a regulation achieving time and a regulation extending range are both approximately ideal.
The second system is a transmission line regulation system. In this system, an input transmission line is regulated from a congestion occurrence point in the upstream direction (input direction), whereby a quasi-congestion occurs in the preceding node or transmission line, and regulation is sequentially extended in the upstream direction to the traffic input point.
The first example of the aforementioned prior art is described with reference to FIG. 1.
Let us consider a packet communication network composed of packet-switching nodes 101.sub.1 to 101.sub.7 for transferring communications #1 to #6 and transmission lines 102.sub.1 to 102.sub.6 for connecting packet-switching nodes 101.sub.1 to 101.sub.7 or the like as shown in FIG. 1. Let us assume that packet-switching node 101.sub.3 detects congestion in transmission line 102.sub.3 (shown by the hatched area in FIG. 1). Node 101.sub.3 detects all communications (communications #1 to #3 in FIG. 1) which pass through transmission line 102.sub.3 or recognizes them beforehand, and must transmit regulation information including the types of communications to packet-switching nodes 101.sub.1, 101.sub.5 and 101.sub.7, which are the sending or source nodes. In this system, however, to specify the input source, the flow of respective communications through respective transmission lines must be watched constantly. Further, to discriminate communications passing through respective transmission lines, call identifying numbers such as the logical channel numbers of individual packets must be detected, and sending nodes of such calls must be detected. As described above, the first example of the prior art has the substantial problem that an enormous hardware and a complicated processing are needed. Thus, this conventional system cannot cope with a future enlarged network.
The second example of the prior art is fundamentally a system for regulating traffic from a transmission line which becomes an input to a transmission line when a congestion occurs in a certain transmission line. This system is described with reference to the packet communication network of FIG. 2, which is similar to that of FIG. 1. If a congestion occurs in transmission line 102.sub.3 accommodated in a certain packet-switching node 101.sub.3, the regulation will be propagated first to transmission line 102.sub.3 ; then to transmission lines 102.sub.2, 102.sub.5, and 102.sub.6 ; and finally to transmission lines 102.sub.1, and 102.sub.4. In other words, although it is originally requested that only communications #1 to #3 be regulated, the regulation is also propagated to communications #4 to #6, thus lowering the throughput of the entire circuit. As described above, the second example of the prior art has the substantial disadvantage that congestion in the entire circuit cannot be alleviated. Furthermore, traffics are stored in the respective stages so that, in addition to the transmitting time, a time in which buffers of respective stages consume is needed also as the regulation achieving time.